A fuel cell is a static energy conversion device in which fuel and oxygen supplied from the outside are electrochemically reacted by a catalytic action and electric energy and heat energy are obtained directly and simultaneously from the fuel. An ion permeable layer called an electrolyte is present between an air electrode (positive electrode) and a fuel electrode (negative electrode) of the fuel cell, and the fuel cell is classified into five types of a phosphoric acid fuel cell (PAFC), a polymer electrolyte membrane (PEM), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), and an alkaline fuel cell (AFC), according to the type of electrolyte.
The fuel cell has an advantage in that the fuel cell has a higher efficiency and thus uses less amount of fuel than existing internal combustion engines, and is a pollution-free energy source which does not generate environmental pollutants such as SOx, NOx, and VOC. Further, the fuel cell has an additional advantage in that a location area required for production facilities is small, a construction period is short, and the like.
Therefore, the fuel cell has various application fields from a mobile power source such as a mobile device and a power source for transportation, such as vehicles, to distributed power generation which can be used for home use and electric power business use. In particular, when the operation of fuel cell vehicles, which are a next-generation transportation device, is put into practical use, a potential market size for the fuel cell is expected to be extensive.
A redox flow battery is one of the rechargeable fuel cells. The redox flow battery has an electrolyte including various kinds of electrically active materials therein, and is a secondary battery in which the charge/discharge occurs due to the oxidation and reduction reactions of the electrolyte. The biggest difference from general batteries is that the charge/discharge occurs while circulating the electrolyte where energy is stored. Specifically, unlike other batteries, an active material of the redox flow battery exists as ions in an aqueous solution state instead of a solid state, and the redox flow battery has a mechanism of storing and generating electric energy according to the oxidation/reduction reaction of each ion in a positive electrode and a negative electrode. That is, the redox flow battery is in an electrolyte liquid (solution) state in which an active material of an electrode is dissolved in a solvent, and when a battery including a catholyte and an anolyte having different oxidation numbers is charged, an oxidation reaction and a reduction reaction occur at the positive electrode and the negative electrode, respectively, and the electromotive force of the battery is determined by a difference between standard electrode potentials (E0) of a redox couple forming the catholyte and the anolyte. Examples of the redox couple include Fe/Cr, V/Br, Zn/Br, Zn/Ce, V/V, and the like but vanadium (V/V) redox couples have been frequently used in consideration of an amount of storable electricity or economic efficiency, and the like. Meanwhile, the electrolyte liquid is supplied from an, electrolyte liquid tank by a pump, and the redox flow battery has both an advantage of a general battery in which the reaction rates of oxidation and reduction are fast on the surfaces of the positive electrode and the negative electrode and an advantage of a fuel cell having high output characteristics.
In order to prepare a separation membrane that permeates only hydrogen, ion exchange resins are used, and when a separation membrane is prepared by using only a polymer resin, the selective permeability of hydrogen ions is low, and the physical strength of the separation membrane is also weak. In order to overcome the aforementioned limitations of the polymer resin, additives are mixed and used to increase the ion conductivity and selective permeability of hydrogen.
However, in the technology of preparing a composite membrane in the related art, an additive is mixed with an ion exchange resin solution, and a film is formed in a form of a single layer by using one solution. The method as described above has a limitation in the amount of inorganic materials added. Further, the method has a disadvantage in that the membrane is split due to additives, which are not dispersed well, because it is difficult to uniformly disperse materials.